C-more to Allen-Bradley PLC Optimization

 Topic: CM631

In most situations, specific steps to optimize the communications between C-more and the Allen-Bradley PLC will not be necessary. Only in applications where very fast performance is desired or larger C-more projects on the bigger panels (15in or 22in) may necessitate more careful planning when designing the system.

To understand how to optimize the communications between C-more and the Allen-Bradley PLC, some fundamental knowledge of how and when communications are initiated is necessary.

Writing Data to the PLC

In most applications, C-more writes Data to the Allen-Bradley PLC infrequently.

  • One way is when a Screen object is user-triggered. A Screen object will only write to the PLC when triggered by the user, such as when a Pushbutton object is pressed or released, the Enter key is pressed in a Numeric Entry object, the Slider object is moved, etc.
  • The second way that C-more will issue a write to the PLC is through the Event manager Tag Copy function. This can be triggered by a value change of a tag, a screen change or time scheduled. See the Event Manager functions for more details.
  • The third way that C-more will write to the PLC is through the Panel To PLC tags. These typically only occur on a value change and happen infrequently most of the time.

All in all, writes occur rarely enough in most cases that they typically do not need to be considered when trying to optimize communications.

Reading Data from the PLC

Several different functions will cause C-more to issue read requests from the PLC.

The most common are the objects on the current screen that is being displayed. The only screen objects that will issue read requests on screens other than the current screen being displayed are the Trend objects. Most of the time, optimizing the PLC addresses assigned to objects on the same screen will provide the most impact to increased performance.

Another function that will issue reads from the panel are Alarm Events and Tag events. Any event created with a tag object addressed to the PLC will constantly read from the PLC no matter which screen is currently being displayed. Careful planning of the addresses used in events will greatly increase communications performance as well.

The PLC to Panel tags will also be read constantly, with no regard to which screen is currently being displayed. PLC addressing of these tags should also be considered.

Optimization for non-Tag-based PLC (SLC and MicroLogix)

The SLC and MicroLogix PLC are fixed addressed PLC. Grouping together consecutive addresses of the same Data type in the same file without significant gaps between the addresses is the only way to optimize communications for these PLC. Putting together tags of consecutive addresses on the same screen, in the event manager or in the PLC-to-Panel screen will greatly reduce the number of requested packets and will therefore greatly increase the number of updates to the objects visible.

Optimization for Tag-based PLC (Control/Compact/FlexLogix PLC)

The Control/Compact and FlexLogix PLC are tag-based PLC. The method of communications (both serial and Ethernet) involves sending the actual symbolic tag name in the request sent over the wire. This means that the tag names themselves affect the speed of the communications by creating larger and/or multiple requests to the PLC. Program space tag names are not efficient. Each tag name needs the PROGRAM: header as well as the name of the program in front of the tag name. This makes the size of the request much, much larger, hence requiring more requests to get multiple tags. Therefore, Controller scope tags should be used as much as possible.

Another method to reduce the size of the tag name requested without necessarily compromising the descriptive nature of a tag name is to use an Alias name in the C-more software. The alias name could be much shorter than the original tag name in the PLC.

The most effective method to reduce the number of requests from C-more to the PLC is to use arrays in the PLC and then assign the C-more tags to consecutive elements in the array of the PLC. C-more can then request multiple PLC tags with one request. This method will, by far, have the most impact in enhanced communication performance between C-more and the PLC.